In a frequency range of electromagnetic radiation of between 1 GHz and 30 THz, also referred as the terahertz frequency range or far infrared frequency or spectral range, many materials like for example paper, cardboard, ceramic and a series of plastic materials are transparent and can be transilluminated. That entails potential applications in particular for imaging systems in quality, packaging and security control as well as for monitoring chemical reactions.
In that respect the frequency of 1 THz corresponds to a wavelength of the electromagnetic wave of 300 μm and a photon energy of 4.14 meV or 33 wave numbers. The radiation is accordingly not ionizing and, unlike for example X-ray radiation, at the other end of the electromagnetic spectrum, does not fall foul of health reservations.
Systems which operate in the stated frequency range of the electromagnetic spectrum however are used not only for the purposes of imaging but also for identifying chemical substances, that is to say for spectrometry. More specifically it has been found that certain chemical substances, for example those which are security-relevant like weapons, explosives or drugs, have characteristic absorption bands in the stated frequency range. If a spectrum, that is to say a frequency-resolved measure in respect of the intensity of the electromagnetic radiation reflected by or transmitted through a chemical substance is successfully recorded in the stated frequency range then that spectrum can be compared to known reference spectra which for example are stored in a database and the substance can be identified.
If such spectrometric methods and spectrometers are used in application environments outside the laboratory then it is necessary for them to operate quickly and also to be operable on the part of non-expert personnel. In addition outside the laboratory effects due to packagings and maskings around the chemical substances which are actually to be detected represent a challenge.